Publisher Summary This chapter describes the various aspects of cereal-legume intercropping systems Intercropping is the growing of two or more crop species simultaneously in the same field during a growing season The intercropping of legumes with cereals offers scope for developing energy-efficient and sustainable agriculture The main types of intercropping include mixed intercropping, row intercropping, and strip intercropping Crop combinations differ with geographical location and there may be intercropping of tree crops, intercropping of tree and field crops, or intercropping of field crops Combinations of crops are determined primarily by the length of the growing season and the adaptation of crops to particular environments Different indices have been suggested for evaluating productivity and efficiency per unit area of land of cereal-legume intercrop systems These include comparisons of absolute yields, protein yields, caloric equivalent, and in economic terms, gross returns from intercrops and sole crops Differences in competitive ability affect the relative performance of component crops and thus the land equivalent ratio values of different cereal-legume intercrop systems It is found that phosphorus is a major nutrient that determines the production potential of most grain legumes usually intercropped with cereals

Cereal–Legume Intercropping Systems

Intercropping, the agricultural practice of cultivating two or more crops in the same space at the same time, is an old and commonly used cropping practice which aims to match efficiently crop demands to the available growth resources and labor. The most common advantage of intercropping is the production of greater yield on a given piece of land by making more efficient use of the available growth resources using a mixture of crops of different rooting ability, canopy structure, height, and nutrient requirements based on the complementary utilization of growth resources by the component crops. Moreover, intercropping improves soil fertility through biological nitrogen fixation with the use of legumes, increases soil conservation through greater ground cover than sole cropping, and provides better lodging resistance for crops susceptible to lodging than when grown in monoculture. Intercrops often reduce pest incidence and improve forage quality by increasing crude protein yield of forage. Intercropping provides insurance against crop failure or against unstable market prices for a given commodity, especially in areas subject to extreme weather conditions such as frost, drought, and flood. Thus, it offers greater financial stability than sole cropping, which makes the system particularly suitable for labor-intensive small farms. Besides, intercropping allows lower inputs through reduced fertilizer and pesticide requirements, thus minimizing environmental impacts of agriculture. However, intercropping has some disadvantages such as the selection of the appropriate crop species and the appropriate sowing densities, including extra work in preparing and planting the seed mixture and also extra work during crop management practices, including harvest. The selection of an appropriate intercropping system for each case is quite complex as the success of intercropping systems depend much on the interactions between the component species, the available management practices, and the environmental conditions. Plant breeding can contribute determinedly to increase of productivity of intercropping systems by investigating and exploiting the genetic variability to intercrop adaptation. This paper provides an overall view and evaluation of annual intercropping, summarizing its main advantages supported by a number of key examples from the literature which point out its great value in the context of sustainable agriculture.

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Annual intercrops: an alternative pathway for sustainable agriculture.

This paper reviews recent research on the processes involved in the yield advantage in wheat (Triticum aestivum L.)/maize (Zea mays L.), wheat/soybean [Glycine max (L.) Merr.], faba bean (Vicia faba L.)/maize, peanut (Arachis hypogaea L.)/maize and water convolvulus (Ipomoea aquatica Forsk.)/maize intercropping. In wheat/maize and wheat/soybean intercropping systems, a significant yield increase of intercropped wheat over sole wheat was observed, which resulted from positive effects of the border row and inner rows of intercropped wheat. The border row effect was due to interspecific competition for nutrients as wheat had a higher competitive ability than either maize or soybean had. There was also compensatory growth, or a recovery process, of subordinate species such as maize and soybean, offsetting the impairment of early growth of the subordinate species. Finally, both dominant and subordinate species in intercropping obtain higher yields than that in corresponding sole wheat, maize or soybean. We summarized these processes as the `competition-recovery production principle'. We observed interspecific facilitation, where maize improves iron nutrition in intercropped peanut, faba bean enhances nitrogen and phosphorus uptake by intercropped maize, and chickpea facilitates P uptake by associated wheat from phytate-P. Furthermore, intercropping reduced the nitrate content in the soil profile as intercropping uses soil nutrients more efficiently than sole cropping.

Using competitive and facilitative interactions in intercropping systems enhances crop productivity and nutrient-use efficiency

The effect of mixed intercropping of field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.), compared to monocrop cultivation, on the yield and crop-N dynamics was studied in a 4-yr field experiment using 15N-isotope dilution technique. Crops were grown with or without the supply of 5 g 15N-labeled N m-2. The effect of intercropping on the dry matter and N yields, competition for inorganic N among the intercrop components, symbiotic fixation in pea and N transfer from pea to barley were determined. As an average of four years the grain yields were similar in monocropped pea, monocropped and fertilized barley and the intercrop without N fertilizer supply. Nitrogen fertilization did not influence the intercrop yield, but decreased the proportion of pea in the yield. Relative yield totals (RYT) showed that the environmental sources for plant growth were used from 12 to 31% more efficiently by the intercrop than by the monocrops, and N fertilization decreased RYT-values. Intercrop yields were less stable than monocrop barley yields, but more stable than the yield of monocropped pea. Barley competed strongly for soil and fertilizer N in the intercrop, and was up to 30 times more competitive than pea for inorganic N. Consequently, barley obtained a more than proportionate share of the inorganic N in the intercrop. At maturity the total recovery of fertilizer N was not significantly different between crops, averaging 65% of the supplied N. The fertilizer N recovered in pea constituted only 9% of total fertilizer-N recovery in the intercrop. The amount of symbiotic N2 fixation in the intercrop was less than expected from its composition and the fixation in monocrop. This indicates that the competition from barley had a negative effect on the fixation, perhaps via shading. At maturity, the average amount of N2 fixation was 17.7 g N m-2 in the monocrop and 5.1 g N m-2 in the intercropped pea. A higher proportion of total N in pea was derived from N2 fixation in the intercrop than in the monocrop, on average 82% and 62%, respectively. The 15N enrichment of intercropped barley tended to be slightly lower than of monocropped barley, although not significantly. Consequently, there was no evidence for pea N being transferred to barley. The intercropping advantage in the pea-barley intercrop is mainly due to the complimentary use of soil inorganic and atmospheric N sources by the intercrop components, resulting in reduced competition for inorganic N, rather than a facilitative effect, in which symbiotically fixed N2 is made available to barley.

Grain yield, symbiotic N2 fixation and interspecific competition for inorganic N in pea-barley intercrops

Wheat/maize or wheat/soybean strip intercropping - I. Yield advantage and interspecific interactions on nutrients

A simple competitive ratio (CR) is proposed as a measure of intercrop competition, to indicate the number of times by which one component crop is more competitive than the other. Intercropping data show that this CR term could be useful in (i) comparing the competitive ability of different crops, (ii) measuring competitive changes within a given combination, (iii) identifying which plant characters are associated with competitive ability, and (iv) determining what competitive balance between components is most likely to give maximum yield advantages.

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A competitive ratio for quantifying competition between intercrops.

Data from 94 experiments on sorghum/pigeonpea intercropping were examined for evidence that the stability of yield is greater with intercropping than sole cropping. Stability of the major component (sorghum) was examined by calculating the distribution of yields; stability of the overall intercropping system was examined by calculating coefficients of variation, by computing regressions of yield against an environmental index, and by estimating the probability of monetary returns falling below given ‘disaster’ levels. All these approaches have some merit; taking the last as an example, it was found that for a particular ‘disaster’ level quoted, sole pigeonpea would fail one year in five, sole sorghum one year in eight, but intercropping only one year in thirty-six. Intercropping gave yield advantages under a wide range of environmental conditions and there was no significant evidence that advantages were greater under stress. This is discussed in relation to possible mechanisms contributing to greater yield stability.

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Evaluation of Yield Stability in Intercropping: Studies on Sorghum/Pigeonpea

Nodulation and N2-fixation of groundnut were investigated in sole and intercropping systems. Intercropping with pearl millet, maize, or sorghum reduced nodulation and N2-fixation. This effect is ascribed to the shading of groundnut by the cereal component and the consequent decrease in photosynthesis of the legume canopy.

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Effect of intercropping on nodulation and N2-fixation by groundnut

This paper tries to illustrate how beneficial interactions between crops can be
exploited to increase the overall output of a cropping system. Growth studies with
sorghum/pigeonpea and millet/groundnut are described to show how intercropping
systems can achieve much larger yields than sole crops by using environmental resources
more fully over time or more efficiently in space. Data from moisture stress studies are
presented to illustrate that these advantages of intercropping can be even greater under
stress conditions.
Possible nitrogen benefits from legumes in intercropping systems are discussed with
particular reference to a study on maize/groundnut. Weed, pest and disease control are
considered and some effects of a sorghum intercrop on the incidence of pod borer and wilt
disease in pigeonpea are described. Evidence for improved yield stability in intercropping
systems is provided from a review of 94 experiments on sorghum/pigeonpea.
It is emphasized that intercropping is especially beneficial to the small farmer in the
low-input/high-risk environment of the developing areas of the world but some brief
comments are made on its applicability in more developed conditions.

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Intercropping studies with annual crops

Various intercrops were examined in an alternate row pattern with pigeonpea or sorghum on both Alfisol (red soil) and Vertisol (black soil). The slow-establishing and later-maturing pigeonpea combined well with earlier cereals and legumes to give very large yield advantages as measured by the Land Equivalent Ratio. In the pigeonpea/cereal combinations, the earlier the cereal the bigger the yield advantage tended to be, attributed to improved use of resources over time as the difference in maturity periods of the component crops increased. Sorghum was generally more competitive than pigeonpea and intercropping advantages tended to be less. But even where there was little difference in maturity periods of the component crops, both sorghum/legume and sorghum/cereal combinations gave substantial and statistically significant advantages, suggesting that improved ‘spatial’ use of resources was also important.

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M. R. Rao

Papers

A competitive ratio for quantifying competition between intercrops.

Evaluation of Yield Stability in Intercropping: Studies on Sorghum/Pigeonpea

Effect of intercropping on nodulation and N2-fixation by groundnut

Intercropping studies with annual crops

Preliminary studies of intercropping combinations based on pigeonpea or sorghum.